1. Field of the Invention
The present invention generally relates to an electrophotographic photosensitive member (e.g., a photosensitive member for electrophotography), and an electrophotographic apparatus.
2. Description of the Related Art
Photosensitive members having a non-single-crystal deposited film that primarily comprises silicon atoms (e.g., an amorphous silicon deposited film including some hydrogen atoms and/or halogen atoms) have been proposed as photosensitive members for electrophotography that are high-performance, highly durable and free of any environmental pollution, and have also been put into practical use.
For the electrophotographic photosensitive members having such an amorphous silicon deposited film, a variety of different kinds of layers have been proposed in accordance with what has been demanded for various performances. Among such various kinds of layers, a surface layer has been recognized as an important layer for achieving various properties such as scratch resistance, light transmission properties and moisture resistance of electrophotographic photosensitive members.
In recent years, as electrophotographic apparatus such as copying machines have become more high-definition, image exposure light of shorter-wavelength has been used. As the image exposure light used is made to be shorter in wavelength, the surface layer of the electrophotographic photosensitive member has also come to require a band gap that is broad enough to transmit short-wavelength light with less absorption.
As a surface layer adaptable to such short-wavelength image exposure light, Japanese Patent Laid-open Application No. 2006-133522 discloses a surface layer formed using an amorphous material composed chiefly of silicon atoms and nitrogen atoms and further containing oxygen atoms and carbon atoms. The surface layer disclosed in this Japanese Patent Laid-open Application No. 2006-133522 has almost no absorption of short-wavelength light of around 380 to 500 nm and has much higher moisture resistance, as noted in Japanese Patent Laid-open Application No. 2006-133522.
Japanese Patent Laid-open Application No. H04-264467 also discloses, as an idea of improving the moisture resistance, a technique in which an electrophotographic photosensitive member is provided with a heater in its interior and the surface of the electrophotographic photosensitive member is heated with the heater at the time of image formation so as to prevent any moisture from being adsorbed to the surface of the electrophotographic photosensitive member, to keep smeared images from occurring.
In recent years, as full-color electrophotographic apparatus become more popular, there has been increasing demand for forming images with higher definition. In particular, image quality equivalent to that of silver salt photography has been increasingly demanded in electrophotography.
The issue of smeared images in a high-humidity environment is considered to be one of the important factors that cause deterioration in the image quality in electrophotography.
The mechanism by which smeared images may occur is also explained in the above Japanese Patent Laid-open Application No. H04-264467. In recent years, it has been pointed out that the smeared images also correlate with the internal binding force of materials used for the surface layer, and defects or imperfections present in the surface layer.
The smeared images may result in damage to dot reproducibility, even if they have occurred at a level where images reproduced are of almost no problem in the usual reproduction of letters or characters. In such a case, gradation tends to be damaged in forming halftone images. This may have a great influence especially in an electrophotographic apparatus in which the image exposure light has been made short-wavelength, where full-color images may become non-uniform in color at their halftone areas. In particular, immediately after the electrophotographic apparatus has been set working, its internal temperature may become unstable, and the smeared images may tend to occur.
In order to keep the smeared images from occurring, in an electrophotographic apparatus which may be used in a high-temperature and high-humidity environment, it has become prevalent to make a heater operate at a low electric power to keep the surface of the electrophotographic photosensitive member preheated.
However, in recent years, with increasing requirements for reduced power consumption, it is becoming more difficult to employ such a heater.
The electrophotographic photosensitive member disclosed in Japanese Patent Laid-open Application No. 2006-133522 is also used in such a service environment, where the heater is switched off and then it is switched on at the same time when the electrophotographic apparatus is set working, and where, under the existing conditions, color non-uniformity due to the smeared images may occur in images reproduced immediately after it has been set working.
As discussed above, there remains a need for an electrophotographic photosensitive member for use in high-definition electrophotographic apparatus in which the image exposure light with short-wavelength light of around 380 to 500 nm is used, with improved performance in the reduction of smeared images (i.e., such that smeared images do not easily occur).